1. Field of the Invention
The present invention relates to a display device in which pixel data for display is written into each pixel arranged in matrix, and to a drive method for the display device.
2. Description of the Related Art
There have been proposed various types of display devices, typified by the one disclosed in Japanese Patent Application Laid-open No. 1998-214060, in which gradation display is performed by dividing each frame period into a plurality of sub-frame periods for driving.
FIG. 1 illustrates a configuration of a circuit of one pixel (pixel circuit) in an organic electroluminescence (EL) display device of time-division gradation display mode disclosed in Japanese Patent Application Laid-open No. 1998-214060. In a simple 2T-1C configuration (including two transistors and one capacitor), when a gate line is at high level, a transistor Tr11 is turned ON to write a data voltage on a data line into a storage capacitor Ch. When the gate line becomes low level and the transistor Tr11 is turned OFF, a transistor Tr12 is driven with the voltage stored in the storage capacitor Ch so that a drive current corresponding to the data voltage flows through an organic EL element EL.
In a normal driving mode, the data line voltage is controlled to control a current of the transistor Tr12, thereby controlling the emission amount (luminance) of the organic EL element EL. Further, in the normal driving mode, the transistor Tr12 is used in the saturation region, and the current flowing through the transistor Tr12 is a current Id, which is determined by a threshold voltage Vth, mobility μ, a gate width W, and a gate length L of the transistor Tr12, as expressed in the following expression.Id=μCo(W/L)(Vgs−Vth)2 where Vgs is a gate-source potential difference and Co is a gate capacitance per unit area. In a thin film transistor (TFT) formed on a glass substrate, especially a low-temperature polysilicon (LIPS) TFT, the values of the threshold voltage Vth and the mobility μ are varied among pixels, resulting in a problem of uneven display.
As a method for solving the problem, the data voltage on the data line is written so that the transistor Tr12 may be completely turned ON to serve as a simple switch (linear region operation) for directly applying “(positive power supply voltage PVDD)-(negative power supply voltage CV)” to the organic EL element, thereby performing gradation display under lighting control with each frame divided into a plurality of sub-frames. FIG. 2 illustrates an example of the lighting method for 4-bit gradation display. As in the example, lighting time is set to T1, T2 (=2T1), T4 (=4T1), or T8 (=8T1) corresponding to each bit.
The voltage-driven type device illustrated in FIG. 1 as the related art has a problem of screen burn-in because luminance becomes lowered as the current amount is reduced by the influence of voltage rise due to the temporal change in the organic EL element. There is another problem that the luminance depends on pixel positions and becomes uneven because of the currents flowing through the organic EL elements in a plurality of pixels as well as voltage drop in power lines. Further, if the number of display gradations is increased, a sub-frame period becomes too short to ensure sufficient write time, which is also a problem.
In view of the above-mentioned problems, as in Japanese Patent Application Laid-open No. 2002-351357, a time-division gradation display device using a current-driven type pixel drive circuit has been proposed. However, such a display device has problems that luminance becomes uneven because of fluctuations in individual current writing and that sufficient write time cannot be ensured.
As exemplified in Japanese Patent Application Laid-open No. 2006-243060, some ideas concerning the problem of the write time inherent in the current-driven type have been proposed. However, there still remains a problem in cost because a multi-bit current source driver has a complicated configuration and it is difficult to set individual current values with accuracy. Further, because the current luminous efficiency of the organic EL element is improving year by year, it is not unusual that a maximum drive current in one pixel of the display device is 1 mA or less. In such a case, a problem of the accuracy in minimum gradation arises.